1. Field of the Invention
The present application relates to cryothermal ablation catheters, as well as to miniature refrigerators and miniature engines for producing mechanical force or effecting mechanical work.
2. Description of the Related Art
Ablation catheters are commonly used to treat arrhythmias by destroying or disrupting cardiac tissue associated with the source of the arrhythmias or their conductive pathways. At present, most ablation procedures are performed using an ablation catheter in which radio frequency (RF) current is passed through tissue contacting the catheter tip to create lesions by means of hyperthermia. Use of such RF current involves risk of char and coagulum formation, particularly if the lesions created are more extensive than focal lesions, such as may be required for circumferential lesions in the pulmonary veins. Formation of char and coagulum is ordinarily caused by poor tissue contact with the catheter tip and creates an undesirable risk of thromboembolic stroke. Other risks of RF ablation include possible unroofing of the endothelium or producing pulmonary vein contraction.
Cryothermal ablation solves many of the problems associated with RF ablation. Destruction of tissue by freezing leaves the connective tissue matrix intact. Lesions are created by rupturing cell membranes, and damaged cells are replaced by fibrotic tissue. There is no formation of char or coagulum, and thus the risk of thromboembolic stroke is low. Additionally, as the tissue is cooled, the catheter tip adheres to the tissue which provides improved stability.
Although cryothermal ablation provides many advantages over RF ablation, it has proven difficult to implement. Cryocatheters today typically comprise a cooling system that provides cooling power by pumping a vaporized refrigerant through a lumen in the catheter to a Joule-Thomson expander located at the catheter distal end. The length of the catheter (often 1 meter or longer) and the small diameter of the lumen within the catheter (often less than 1 mm in diameter) limit the flow rate through the Joule-Thomson expander. Additionally, the pressure of the vapor returning through the catheter must be held under 1 atmosphere to meet FDA requirements, which further limits the flow rate through the Joule-Thomson expander. The cooling power of the system consequently is limited to about 2 Watts, which limits the depth of tissue ablation to about 4 mm.
Accordingly, there is a need in the art for a cryocatheter that does not require transport of refrigerant along the length of the catheter so as to permit increased refrigerant flow rates. At a more fundamental level, there is a need for a miniature engine that can be adapted to, among other things, drive a miniature refrigeration system in the tip of a cryocatheter.